Conduit Fill Calculator — NEC Chapter 9, EMT and PVC

Why the limit is 40% and not 90%

The empty space in a conduit is not wasted. Pulling a conductor around bends creates friction, and if too much of the cross-section is occupied the insulation is abraded or the conductor simply will not move. The 40% ceiling exists to keep pulling tension within the range where a normal installation does not damage the wire.

This is why the fill percentage is calculated on the internal cross-sectional area of the raceway, not on its outside diameter. A conduit described as "trade size 3/4 inch" has an internal area of 0.533 square inches in EMT — considerably less than the roughly 0.44 square inches a 3/4 inch circle would suggest if you mistakenly used the nominal dimension.

Fill limits at a glance

Note the counter-intuitive result: two conductors are allowed less fill than three. Two round conductors in a round raceway wedge against each other and cannot slide past one another. A third conductor gives them somewhere to go. Installers who assume "fewer wires means more room" get this backwards.

Counting conductors correctly

• Count every current-carrying conductor. That includes the grounded (neutral) conductor on most circuits. It does not include a bare or green equipment grounding conductor, which is counted separately by the fill calculation but is still a physical body in the raceway.
• A neutral used only for a purely 240 V load such as a water heater is often not counted as current-carrying.
• Metallic raceways. An EMT conduit is itself the equipment grounding conductor when installed per Article 358, so no green wire is required. PVC is not, so a separate grounding conductor is needed.
• Insulation type matters. THHN and THWN-2 are much slimmer than older TW or RHW insulation. The same raceway holds far fewer TW conductors, which is why fill tables are indexed by insulation type as well as size.

Two worked examples

1. Four 12 AWG THHN conductors in 3/4 inch EMT

Each 12 AWG THHN conductor has an area of about 0.0133 square inches, so four of them occupy 0.0532 square inches. A 3/4 inch EMT has an internal area of 0.533 square inches. The fill is 0.0532 divided by 0.533, or about 10%. That passes the 40% limit by a very wide margin — which is why 3/4 inch EMT is the standard choice for a 20 A circuit run. You could fit sixteen conductors of that size before reaching 40%.

2. Three 4/0 conductors plus a ground in 3 inch EMT

4/0 THHN has an area of about 0.3237 square inches. Three current-carrying conductors give 0.9711, plus an 6 AWG ground at 0.0507 gives 1.0218 square inches. A 3 inch EMT has 8.846 square inches internally, so the fill is about 11.6%. This example shows why large commercial feeders rarely challenge the fill limit: the conductor count is small even though each conductor is large.

Fill is not the only check

Passing the fill calculation does not mean the installation is compliant. Also verify:

• Ampacity adjustment for bundling. More than three current-carrying conductors in one raceway requires derating under Table 310.15(C)(1) — 80% for four to six conductors, 70% for seven to nine. A conduit that passes the fill test can still fail on ampacity.
• Conduit support spacing. EMT must be supported within 3 feet of each box and at intervals not exceeding 10 feet.
• Bend limits. No more than 360 degrees of total bend between pull points.
• Minimum size. Raceways must be at least trade size 1/2 inch for most wiring.

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